1. Field of the Invention
This invention relates to an improvement of a speed-shifting device including variable speed pulleys to change a transmission ratio of a pair of transmission shafts by varying the effective radius of the pulleys for a belt member.
2. Description of the Prior Art
In agricultural implements and the like, many transmission type speed-shifting devices have been used which have a belt drive for transmitting the power of an engine to a driven wheel or an operating device which requires changing speed. As a belt transmission type speed-shifting device utilizing a belt drive, a variable speed type device is well known, for example, comprising a pair of parallel transmission shafts, variable speed pulleys composed of fixed sideplates fixedly rotatably and axially non-slidably mounted on each transmission shaft forming a V-shaped groove with the fixed sideplates facing thereto, and a belt member trained in the V-shaped grooves between the respective pairs of sideplates, wherein the transmission ratio between the two transmission shafts is adjusted by changing the effective radius of the pulleys by moving axially the movable sideplates of each variable pully toward and away from the fixed sideplates.
In such a speed-shifting device, generally a variable speed pulley on one of the transmission shafts is a driving pulley, the movable plate of which is axially moved by a driving mechanism utilizing, for example, hydraulic equipment or a cam mechanism, to change the effective radius of the driving pulley, and the variable speed pulley on the other transmission shaft is a driven pulley, the effective radius thereof adjusted in dependence on the movement of the belt member by change of the effective radius of the driving pulley under the bias of a spring urging the movable sideplate toward the fixed sideplate.
However, there occur various problems as described below resulting from the high bias force on the movable sideplate in the driven pulley. Namely:
(1) Since the tension of the belt member trained between the two pulleys is substantially set by the spring adapted to urge the movable sideplate of the driven pulley toward the fixed sideplate, it is necessary to increase the bias force on the movable sideplate by increasing the force of the spring in order to ensure the power delivery between the transmission shafts. Since the cost of the high force spring is high, there is the problem of an increase in the total cost. In addition to this problem, when assembling a speed-shifting device, setting the spring thereinto is not easily done.
(2) Also, since the speed-shifting process is such that when the effective radius of said driving pulley is changed by the movement of the movable sideplate, the belt member moves in accordance therewith, with the result that the change of the effective radius of the driven pulley is varied and the transmission ratio between the transmission shafts varies thereafter, so that a certain period of time is required for shifting the speed. Particularly, when the transmission ratio is shifted from a low speed mode to a high speed mode, a high handling force is required to move the movable sideplate of the driven pulley away from the fixed sideplate against the bias of the spring. As a result, it is hard to shift the transmission ratio between the transmission shafts quickly and smoothly with a small handling force.
(3) Moreover, as shown in FIG. 9, when the driving mode changes from the unloaded mode, in which no driving power is transmitted between the transmission shafts (a) and (b), into the load mode in which a certain driving power is transmitted, the pitch line of the belt member (c) located at the position shown in full lines in this figure in the unloaded mode of the driven pulley (d) moves to a middle position thereof shown in a broken line by an inner thrust of the belt member (c). Nevertheless, in the driven pulley (e) the movable sideplate (e1) is urged by the spring. Therefore, its pitch line moves radially outwardly attended with an elongation of the whole belt member (c) and a decrease in the effective radius due to the inner thrust of the belt member (c) in said driving pulley (d), resulting in a certain decrease in rotational speed at the output transmission shaft.
In order to solve the abovementioned problem, the objects of this invention are to reduce the force of the spring for giving a tension to the belt member, to smooth the speed shifting operation by making it such as to require only a slight handling force, and to prevent decrease in the rotational speed of the driven shaft caused by the elongation of the belt member during the shifting of the drive mode from the unloaded to the loaded mode by providing a means for moving the movable sideplates of both pulleys synchronously and by giving the tension to the belt member by other means than the bias of the movable sideplate toward the fixed sideplate of the driven pulley.